Electrophotographic photoreceptors using an organic pigment as a photoconductive substance are being given much study for further developments because of their advantages over those using inorganic substances from the standpoint of easy processing of materials, a wide choice of materials, low cost, and freedom from necessity of recovery.
The organic photoreceptors include those comprising a photoconductive compound and a sensitizer, those comprising a photoconductive compound of charge transfer complex type, and those having a charge generating layer and a charge transporting layer, called separate function type photoreceptors. The separate function type photoreceptors are now taking the lead among organic photoreceptors on the market because of a wide choice of materials allowed and ease in designing of devices.
Methods for forming a charge generating layer of the separate function type photoreceptors are generally divided into a coating method comprising coating an organic pigment dispersion in a binder resin on a substrate and a deposition method comprising forming a thin deposit on a substrate by vacuum deposition.
However, in a resin dispersion type charge generating layer formed by the coating method, movement of charge carriers generated in the organic pigment particles is hindered by resin molecules among the pigment particles, resulting in impairment of photoconductivity essentially possessed by the pigment, thus failing to exhibit satisfactory sensitivity. On the other hand, containing no binder resin, a deposited charge generating layer formed by the deposition method does not suffer from such a hindrance to charge carrier movement and exhibits satisfactory photoconductivity, giving an assurance of high sensitivity. However, the state-of-the-art electrophotographic photoreceptors having the deposited charge generating layer are still unable to sufficiently satisfy other performance characteristics required, such as high chargeability, small dark decay, low residual potential, and durability, e.g., potential stability on repeated use.
It is known that impurities contained in organic pigments have influences on electrophotographic characteristics of electrophotographic photoreceptors. In the case of the resin dispersion type charge generating layer, electrophotographic characteristics are maintained within a given range on an account of the presence of impurities in the organic pigment used. Therefore, organic pigments purified by general techniques or commercially available organic pigments can be used as they are. However, if such organic pigments are employed as a source of evaporation in the formation of a deposited charge generating layer, electrophotographic characteristics of the resulting electrophotographic photoreceptor vary to a large extent and are deteriorated. While it is thus highly necessary to control impurities in the organic pigments for use as an evaporation source, the impurity control alone is not sufficient for assuring stable electrophotographic characteristics.